The Heart as a Muscular Pump

Question 1

True or flase: one can tetanize the heart?

Answer 1

False
There is no summation of contractile events in cardiac muscle
Due to L-type Ca++ Channel

Question 2

What is the major mechanisms of skeletal muscle contraction that is absent in heart muscle?

Answer 2

Vary the number of active motor units
Each myocyte is connected via gap junctions
They all contract together

Question 3

Greatest way to change contractility

Answer 3

Change Ca++

Question 4

Force-Velocity Relationship of the Heart

Answer 4

Vo = Maximum cycling rate (i.e. no load)
Po = Dependent upon Ca ++ no shortening or lengthening

Question 5

Law of LaPlace

Answer 5

Stress = (Pressure * Radius)/ (Wall Thickness)

Question 6

When is maximal work done by the ventricle?

Answer 6

When the ventricle ejects against a moderate volume at a moderate pressure
When a ventricle contracts against high pressure, it does no work because there is zero shortening
When the ventricle ejects against a low pressure, there is no work done since the force in the walls is zero
Stroke Work = Pressure * Stroke Volume

Question 7

Most Demanding type of work

Answer 7

Internal work

Question 8

Determinants of Preload

Answer 8

1. Venous Return
2. End systolic (residual) volume
3. Diastolic stiffness (lusitropy)

Question 9

Determinants of Afterload

Answer 9

1. Aortic Pressure

2. Contractility (inotropy)

Question 10

Definition of Afterload

Answer 10

pressure at which the aortic valve opens

i.e. the pressure at which the left ventricle “meets its afterload”